Abiotic clathrite synthesis from CO2-clathrate under ocean world conditions

Introduction

A planet that harbor water in a liquid state becomes as a study subject for its habitability evaluation. Water is the only known solvent in which the reactions for life as we know it take place, while is also a sink of molecules that include bio-essential elements such as carbon.

On Earth, oceans are reservoirs of CO₂ and CH₄ and, along with other molecules (H₂S, N₂, etc.) may be encapsulated in minerals of clathrate hydrates (thereafter clathrates) under high pressure and low temperature conditions [1]. Water molecules begin to arrange in space and join through hydrogen bonds, constructing a tridimensional crystal network that host gases inside by the van der Waals interaction forces [2]. This physical and chemical conditions required for clathrate deposits formation are found on Earth in continental margins and polar regions [3].

On Europa and Enceladus moons, clathrate deposits would not only be found in the seafloor, but also floating into oceans [4, 5, 6]. On Titan, as well as on Europa, they might form part of the composition of water-ice crust [7, 8], whereas on Ganymede and Pluto they may constitute some global layers of its internal structure [9, 10].

Clathrate formation and dissociation would play a role in geological processes and, above all, in (bio)-geochemical cycles of these planetary bodies. As clathrates are sinks of carbon and other chemical elements essential for life, the dissociation of these minerals by changes in physical chemical conditions would release gases into Europan and Enceladus´ oceans, enabling to promote favourable conditions for development of a hypothetical chemolithoautotrophic life [11]. However, gases could also be sequestrated again as carbonates or another salts due to reactions between them and rocky core in the water-rock interphase.

On Earth, encapsulated gases into clathrate structure may be metabolized by organotrophs [15] and/or by a consortium of methanotrophic archaea and sulfate-reducing bacteria after its dissociation [11, 16]. As a consequence carbonate precipitates, known as clathrite [17] because it records the past presence of these deposits. The aim of this study is to simulate the abiotic clathrite formation process under ocean-world-environmental-conditions when there are calcium saturation during clathrate formation and dissociation.

 

Methodology

For the experiments, we used a high-pressure simulation chamber made of stainless steel (volume capacity 67 ml) which is connected to a tank of CO₂ (gas). It is coupled with a thermocouple and pressure sensor to monitor temperature and pressure parameters and with a Raman spectrometer to analyse phase changes. We filled the high-pressure cell with crushed ice made of 7.4 wt% Ca(OH)₂ dissolution. The chamber was pressurized at 30 bar and then temperature was reduced down to 260 K. Once CO₂ clathrates were formed, the chamber was heated slowly up to 284 K. We studied the synthesis process of clathrite, taking in situ Raman spectra with a 532 nm laser at every pressure and temperature change.

 

Results

Carbonate precipitation occurred since CO₂ was injected to the chamber. The final product phase obtained was calcite. Nevertheless, during experiment of clathrate formation and dissociation, carbonate structure took diverse polymorphs of calcium carbonate different from pure calcite, aragonite and vaterite structures. This was evidenced by the spectral signature within the ranges of 1069.42-1087.75 cm^-1 and 709.26-731.94 cm^-1 for stretching and bending vibration of the CO₃^2- ion respectively and 150.30-160.62 cm^-1, 194.17-211.97 cm^-1 and 280.03-289.94 cm^-1 for lattice modes.

 

Acknowledgments

We thank project PID2019-107442RB-C32 funded by MINECO. Ana de Dios is supported by the AEI pre-doctoral contract under the project MDM-2017-0737-19-1.

 

References

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